1. Field of the Invention
The present invention relates to a power supply apparatus provided with a regulation function for stabilizing an output voltage.
2. Description of the Related Art
Small-sized information terminals such as a cell phone and a personal digital assistant (PDA) incorporate a device, such as a light emitting diode used as backlight for a liquid crystal display, which requires a voltage higher than an output voltage of a battery. For example, Li-ion batteries are often used in these small-sized information terminals. The output voltage of a Li-ion battery is usually about 3.5V and, even when fully charged, is about 4.2V. Meanwhile, an LED requires a voltage of about 4.5V for driving. When a voltage higher than a battery voltage is required, a power supply apparatus using a booster circuit of a charge pump type is employed to boost the battery voltage and output a voltage necessary to drive the LED.
In the power supply apparatus provided with the booster circuit, overshoot defined as a transient change in an output voltage to a greater value occurs at start-up or upon occurrence of a significant load change. An overshoot may sometimes adversely affect the reliability of a device such as an LED connected as a load to the power supply apparatus.
In the related art, soft start, in which an output voltage is slowly brought to a desired level at start-up, has been known as a means to prevent overshoot (see patent documents No. 1 and No. 2).
[Patent Document No. 1]
    JP 2002-369501 A[Patent Document No. 2]    JP 2003-339156 A
Some power supply apparatuses provided with a booster circuit are configured such that the charge pump gain of the booster circuit is made to vary so that a constant voltage is obtained even when a battery voltage, an input voltage, drops. In the booster circuit built in the apparatus of this type, an overshoot beyond a preset output voltage temporarily occurs not only at start-up but also when the charge pump gain is switched. Given that the voltage input to the booster circuit is 3V, an overshoot of the output voltage to 6V may occur when the charge pump gain is switched from 1.0 to 2.0.